fix!: Remove Ruby 2.3 from CI matrix

cairo depends on unspecified version of red-colors, which uses `Comparable#clamp`, that is not supported
by ruby 2.3. We could lock red-colors manually, but ruby 2.3 causes more issues with the soon-incoming
changes, eg "undefined symbol: RB_NIL_P" when installing glib2-4.2.2

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.3, 2.7]
+        ruby: [2.7]
     name: install dependencies and run tests
     steps:
       - name: install image processing dependencies

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ## Unreleased
 ### Removed
 - support for Ruby 2.0 (and illusion of it being tested by CI)
+- support for Ruby 2.3
 
 ## [0.13.0] 16.12.2020
 ### Fixed